In the context of optical storage and optical microscopes, U.S. Pat. No. 5,265,617 granted on Apr. 27, 1997 to L. C. Hopkins et al. (hereinafter referred to as Hopkins) describes near-field optical detection apparatus that includes a semiconductor laser having a non-uniform emission face (e.g., output facet) configured so that at least 50% (preferably 90-95% of the total radiation emission from the output facet is emission from a first region having a width w&lt;.lambda..sub.s, where .lambda..sub.s is the center wavelength of the laser emission at the output facet. Illustratively, the emission face has a coating thereon, and an aperture, recess, protrusion or other structural feature (hereinafter referred to as a feature) is formed in the coating. Alternatively, as pointed out at column 7, line 59-67, the feature may be formed directly in an (uncoated) output facet. Depending on the relative reflectivity of the feature compared to the surrounding areas of the facet, the first region may be located either at the feature or in the surrounding area. Hopkins contemplates embodiments with both edge-emitting semiconductor lasers (e.g., ridge waveguide lasers) and surface-emitting semiconductor lasers (e.g., VCSELs). This patent is incorporated herein by reference.
As noted at column 6, lines 55-57 of Hopkins, it is typically desirable to position the feature (e.g., a recess in the output facet coating) substantially in the center of the active region of the laser. In the case of edge-emitting ridge waveguide lasers, this desideratum means that the feature is typically positioned at the center of the cross-section of the waveguide.
The utilization of such a laser to read information contained at the data sites (i.e., bit marks) of a storage medium is described by Hopkins at column 10, lines 32-67. Briefly, variations in the reflection properties of the data sites result in variations in at least one laser parameter (e.g., laser optical output intensity, and/or laser terminal voltage). In an exemplary embodiment, these variations in the radiation reflected by the data sites back into the laser result in corresponding variations in the intensity of radiation emitted from the back facet of the laser, which can be detected in conventional fashion by means of a photodetector. Alternatively, the Hopkins optical storage apparatus may be operated in a transmission mode as shown and described with reference to FIG. 11 therein.
The Hopkins laser and optical storage apparatus do not, however, rely on the laser operating in multiple transverse modes, and in particular, do not exploit optical filaments formed by the coupling of such transverse modes to one another.
With respect to solid state lasers that include a planar optical waveguide, the term filament as used herein means an intracavity, in-plane (i.e., in the plane of the waveguide) intensity distribution of the lasing radiation that exhibits a meandering (e.g., sinusoidal) pattern of nodes and peaks that weaves from one side of the waveguide cross-section to another (or from the top to the bottom of the waveguide cross-section) along the longitudinal axis of the laser. A few prior art lasers have exhibited such filaments; e.g., 0.98 .mu.m pump laser diodes and 1.3 .mu.m buried heterostructure (BH) laser diodes investigated by Ohkubo et al., Jpn. J Appl. Phys., Vol. 35, pp. L34-L36 (1996) and Schemmann et al., Appl. Phys. Lett., Vol. 66, No. 8, pp. 920-922 (1995), both of which are incorporated herein by reference. But prior art workers have considered filamentation in these lasers to be undesirable because the maximum useful output power is limited by the lateral beam deflection that occurs when the filament forms. In addition, the authors did not appreciate the way such filaments might be used in optical detectors.
On the other hand, in the context of optical receivers for use, for example, in optical communication systems, information (e.g., data, voice, video) carried on a laser beam is typically detected by a p-i-n photodiode. In such devices absorption of the laser beam in the i-region generates electron-hole pairs. Under reverse bias electrons and holes swept out of the i-region generate a photocurrent that corresponds to the intensity of the laser beam (e.g., to the intensity of the pulses in a digital system). The maximum speed (i.e., data rate in a digital system) at which the system can operate is determined, in part, by how fast carriers can be swept out of the i-region of the photodiode. Typically the maximum speed is on the order of a few 10s of GHz.
There is a need in the optical communications systems industry, however, for systems and hence photodetectors that can operate at much higher speeds; e.g., on the order of 100s of GHz.